1. Field of the Invention
The present invention relates to processes for producing a pigment dispersion and an ink composition.
2. Background Art
Pigments have been used as colorants in highly lightfast and highly waterfast ink compositions, especially ink compositions for ink jet recording. When a pigment is used as a colorant, an ink composition is prepared by mixing and dispersing the pigment, together with a dispersant, in water and the like to prepare a pigment dispersion and further mixing the pigment dispersion with water, a water-soluble organic medium and other ingredients.
For example, Japanese Patent Laid-Open No. 157954/1994 proposes an ink production process in which a pigment and a polymeric dispersant are subjected to a two-roll milling (a two-roll mill) device to prepare a pigment dispersion and then the pigment dispersant is dispersed in an aqueous carrier medium to give an ink composition. Japanese Patent Publication No. 55548/1986 also proposes an ink production process in which a polymer (a dispersant) previously prepared by polymerizing monomers is first provided, and the polymer, a pigment, and a water-soluble polyhydric alcohol (ethylene glycol) are dispersed with a ball mill or a sand mill to prepare a pigment dispersion from which an ink composition for ink jet recording is then prepared. Japanese Patent Laid-Open No. 227668/1992 proposes a production process of an ink composition. In this production process, a previously prepared block polymer is provided as a dispersant. The dispersant, a pigment, a water-soluble organic solvent, such as diethylene glycol, and water are dispersed with a horizontal Mini mill to prepare a dispersion which is then diluted to prepare an ink composition. Further, WO 98/44059 proposes an ink production process in which ingredients of a mixture containing a pigment, a dispersant, a specific alkyl ether compound, and a specific surfactant are dispersed in one another to prepare a pigment dispersion. On the other hand, a production process of a pigment dispersion, wherein ingredients of a mixture of a pigment, a dispersant, a polysiloxane surfactant and/or an alkanediol, a polyhydric alcohol, and water are dispersed in one another, has not been disclosed.
The present inventors have now succeeded in developing a production process of a pigment dispersion that can realize an ink composition which can yield good images. In particular, the present inventors have found that a pigment dispersion wherein, in dispersing a pigment with the aid of a dispersant, a specific compound is added to provide a pigment dispersion that can realize an ink composition which can form dots having a large diameter in a small amount of ink ejected on a recording medium and can effectively prevent feathering.
Accordingly, an object of the present invention is to provide a production process of a pigment dispersion that can realize an ink composition which can form dots having a large diameter in a small amount of ink ejected on a recording medium and can effectively prevent feathering.
According to one aspect of the present invention, there is provided a process for producing a pigment dispersion, comprising the steps of:
providing a mixture consisting essentially of a pigment, a dispersant, a polysiloxane surfactant and/or an alkanediol, a polyhydric alcohol, and water; and
dispersing the mixture.
According to another aspect of the present invention, there is provided a process for producing an ink composition, comprising the steps of:
providing a mixture consisting essentially of a pigment, a dispersant, a polysiloxane surfactant and/or an alkanediol, a polyhydric alcohol, and water;
dispersing the mixture to prepare a pigment dispersion; and
mixing the pigment dispersion with a member selected from the group consisting of water, a surfactant, a wetting agent, a pH adjustor, and mixtures thereof.
According to the present invention, in dispersing a pigment with the aid of a dispersant, a polysiloxane surfactant or an alkanediol is added. An ink composition using the pigment dispersion thus obtained, as compared with an ink composition prepared by dispersing a pigment with the aid of a dispersant to prepare a pigment dispersion and adding a polysiloxane surfactant or an alkanediol to the pigment dispersion, can advantageously yield dots having the same dot diameter in a smaller amount of ink ejected and can realize images having no significant feathering on recording media. In particular, the pigment dispersion according to the present invention is advantageous in that, when the recording medium is a gloss film, that is, a film comprising a white film as a substrate and a transparent coating provided on the surface of the substrate, dots having a relatively large diameter can be realized in a small amount of ink ejected.